1. Field of the Invention
The present invention relates to an optical head device, a recording and/or playback apparatus and a recording and/or playback method in which a thickness error of the light transmission layer of an information recording medium for performing recording or reproduction of information is detected to correct spherical aberration generated due to the error.
2. Description of the Related Art
In an optical head device used in a recording and/or playback apparatus, there is a great demand for increasing the numerical aperture (NA) of the objective lens to record information with higher density. When the numerical aperture is increased, it becomes possible to reduce the diameter of the condensing spot. At the same time, however, the error degree allowed for the system decreases. More specifically, the permissible error degree regarding the thickness of the light transmission layer of the information recording medium, the permissible error degree regarding the inclination of the information recording medium with respect to the objective lens, decrease. Here, the spherical aberration, which is predominant in an optical system in which the NA is large (e.g., 0.85), will be explained.
As stated in “Optical Disk Technique” (published by Radio Gijutsusha), pp. 60 to 62, the spherical aberration W40d generated due to the thickness error Δd of the light transmission layer can be expressed by equation (1), using the refractive index n of the light transmission layer material and the numerical aperture NA of the objective lens.W40d=(n2−1)/(8×n3)×(NA)4×Δd  (1)
For example, as can be seen from equation (2), an optical system using an objective lens having an NA of 0.85 has to be approximately four times as accurate as an optical system using an objective lens of an NA of 0.6 such as DVD in terms of the thickness error of the light transmission layer.
                                                                                          W40d                  ⁡                                      (                    0.85                    )                                                  /                                  W40d                  ⁡                                      (                    0.6                    )                                                              =                                                (                                      0.85                    /                    0.6                                    )                                4                                                                                        =              4.02                                                          (        2        )            
As can be seen from the above, detecting the thickness error of the light transmission layer and performing some sort of correction on the spherical aberration is an effective means of realizing high-density recording/playback. In this regard, there has been proposed an optical head device which has a first optical system for detecting the thickness error of the light transmission layer of an information recording medium and a second optical system for performing recording/playback of information and in which the position of an optical element in the second optical system is adjusted so as to correct the thickness error detected by the first optical system.
FIG. 14 shows a conventional optical head, which is disclosed in Japanese Unexamined Patent Application Publication No. 2000-11402. In the drawing, numeral 1 indicates an optical head, numeral 2 indicates an optical disk, numeral 3 indicates a substrate, numeral 4 indicates a light transmission layer, numeral 5 indicates a first optical system, and numeral 6 indicates a second optical system. Numeral 7 indicates a light source, numeral 8 indicates a beam splitter, numeral 9 indicates a collimator lens consisting of two spherical lenses 9a and 9b glued to each other, numeral 10 indicates a hologram element, numeral 11 indicates an objective lens, and numeral 12 indicates a photo detector having a first light receiving portion 12a and a second light receiving portion 12b. Numeral 13 indicates a light source, numeral 14 indicates a cylindrical lens, numeral 15 indicates a polarization beam splitter, numeral 16 indicates a collimator lens consisting of two spherical lenses 16a and 16b glued to each other, numeral 17 indicates a diffraction grating, numeral 18 indicates a boot-up mirror, numeral 19 indicates a ¼ wavelength plate, numeral 20 indicates a 2-group objective lens consisting of two spherical lenses 20a and 20b glued to each other, numeral 21 indicates a photo detector, numeral 22 indicates an actuator, numeral 23 indicates a condensing lens, numeral 24 indicates an output detection photo detector, and numeral 30 indicates a 2-axis actuator.
In the above construction, the laser beam emitted from the light source 7 is reflected by the beam splitter 8, converted to parallel rays by the collimator lens 9, diffracted by the hologram element 10 and separated into 0-order light and first-order light having different focal positions. Then, the 0-order light and the first-order light are condensed on the optical disk 2 by the objective lens 11. The focus of the 0-order light differs from that of the first-order light by a length substantially equal to the thickness of the light transmission layer 4, so that the 0-order light is transmitted through the light transmission layer 4 and condensed on the recording layer to form a spot, the first-order light being condensed on the surface of the light transmission layer 4 to form a spot.
Next, the 0-order light and the first-order light reflected by the optical disk 2 travel by way of the former optical path and are transmitted through the objective lens 11 before they are converted to convergent light by the collimator lens 9, impinging upon the first light receiving portion 12a and the second light receiving portion 12b on the photo detector 12. First, the first light receiving portion 12a detects a focus error signal due to the return light from the recording layer of the optical disk 2, and the second light receiving portion 12b detects a focus error signal due to the return light from the surface of the light transmission layer 4 of the optical disk 2. Astigmatism is imparted to these return lights by the beam splitter 8 arranged in the convergent light, and the focus error signals are detected by means of the well-known astigmatism method.
On the other hand, in the second optical system 6, the laser beam emitted from the light source 13 is beam-shaped by the cylindrical lens 14, and is then transmitted through the polarization beam splitter 15 before it impinges upon the collimator lens 16. The laser beam output from the collimator lens 16 impinges upon the diffraction grating 17, is diffracted into 3 beams. Then, the proceeding direction is bent by the boot-up mirror 18 before the beam impinges upon the ¼ wavelength plate 19. The laser beam output from the ¼ wavelength plate 19 impinges upon the 2-group objective lens 20, and is condensed on the recording layer of the optical disk 2. The return light reflected by the recording layer travels by way of the former optical path and is turned into convergent light by the collimator lens 16 before it is reflected by the polarization beam splitter 15 and impinges upon the photo detector 21 with the result that a signal is detected.
Numeral 22 indicates an actuator. By moving the collimator lens 16 in the optical axis direction of the laser beam based on the focus error signal detected by the second light receiving portion 12b in the first optical system 5, it reduces the spherical aberration of the light spot condensed on the medium surface of the optical disk 2. In the condition in which the light source 13 is arranged at the focal position of the collimator lens 16, parallel rays are output from the collimator lens 16, and impinge upon the 2-group objective lens 20, so that no spherical aberration is generated.
However, when the collimator lens 16 is moved in the optical axis direction of the laser beam, the laser beam transmitted through the collimator lens 16 ceases to parallel rays, and the light impinges upon the 2-group objective lens 20, with the result that spherical aberration is generated. Utilizing this nature, the amount and polarity of the spherical aberration generated in the light spot condensed on the medium surface of the optical disk 2 are detected, and the distance between thee light source 13 and the collimator lens 16 is varied so that a spherical aberration of the opposite polarity may be generated. The condensing lens 23 condenses the laser beam reflected by the polarization beam splitter 15 on an output adjusting photo detector 24. On the basis of the light reception amount of the output adjusting photo detector 24, the output of the laser beam from the light source 13 is automatically adjusted. Numeral 30 indicates a 2-axis actuator having a movable portion on which the objective lens 11 and the 2-group objective lens 20 are mounted, focusing control and tracking control of the objective lens 11 and the 2-group objective lens 20 being performed.
In the above-described optical head device, the optical system (first optical system 5) for detecting the thickness error of the light transmission layer and the optical system (second optical system 6) for performing the recording/playback of information are separately provided, resulting in a complicated construction, an increase in production cost and a large apparatus size.
Further, on the optical disk, the position where the thickness error of the light transmission layer is detected is in the optical axis of the objective lens 11, and the position where the recording/playback of information is performed is in the optical axis of the 2-group objective lens 20, which means they are spaced apart from each other, and the detection of the thickness error at the position where the recording/playback of information is performed cannot be effected accurately. The distance might be chronologically compensated for. However, that would involve provision of an excess circuit.